An electrically powered propulsion system is a form of electric propulsion used for spacecraft propulsion that creates thrust by accelerating ions or plasma. Because the total thrust of a single electric thruster (thruster) is low, an electrically powered propulsion thruster (EP thruster) for a spacecraft consists of the plurality of single thrusters. In addition to the increased total thrust some redundancy can be achieved.
For propulsion systems that are meant to impart delta v as opposed to torques, the total thrust vector, which is defined as the geometric addition of all thrust vectors of all individual thrusters, must pass through the spacecraft center of gravity (CoG). Typically, the plurality of thrusters is oriented in parallel. This assures that the resulting force passes through the spacecraft center of gravity, both at the beginning of life (BoL) and at the end of life (EoL). However, there are several issues which complicate the requirement that the total thrust vector passes through the center of gravity. First, an initial thruster alignment may be imperfect. Second, the thrust vector may deviate from the center axis of the thruster, both initially and over time. Third, the thrust vector may be influenced by external factors, such as an earth magnetic field or a magnetorquer field. Fourth, the center of gravity moves over time as a propellant of the electric propulsion thruster is being depleted. Fifth, one or more thrusters may fail during operation.
Although the above stated problems affect all kind of propulsion systems, it is to be noted that its solution is most urgent in the case of electrical propulsion. Standard chemical propulsion systems provide high force and thrusters are relatively inexpensive. As a result, any offset of the total thrust vector from the center axis of the thruster is usually compensated by impulses from additional thrusters.
However, this approach used for chemical propulsion systems cannot be transferred to electric propulsion systems due to its low thrust and very high cost per thruster.
To match the total thrust vector with the center of gravity of the spacecraft, thruster pointing mechanisms may be used. However, thruster pointing mechanisms are heavy, difficult to test, expensive and take up significant space. Alternatively, it is known to use an internal balance mass, e.g. on a spindle, to adjust the center of gravity. However, with a balance mass only very minor shifts can be achieved. Considerations to accept some offset between the total thrust vector and the center of gravity lack from the fact that it is difficult to compensate the resulting torque. However, the resulting torque builds up very quickly.
One possible mitigation is a 180° roll maneuver of the spacecraft, which balances out the influence of the offset. However, this takes a significant amount of time, during which the power generation of the array is impaired.
Furthermore, the offset between the total thrust vector and the center of gravity may be counteracted by wheels which need to be de-saturated somehow. This might be done with the help of magnetorquers. However, these are only efficient in low earth orbits (LEO), while most applications for electric propulsion systems either are in medium or geostationary orbits (MEO, (GEO) or during the transfers there.